Until now, electrical connections were the main connections used in printed circuit boards. Electrical connections are produced in and on insulating layers according to a technologically well-controlled process and interconnected via soldered and plug connections. During this process, solder terminals are used to interconnect integrated circuits in particular, which are mounted in a housing, which in turn has contact points corresponding to contact points on the printed circuit board. Although initially a raster of 1/10 inch, approx. 2.5 mm, was customary, it has since become possible to achieve a finer raster using precision conductor technology, without significantly altering the order of magnitude. When positioning components on the printed circuit board, positioning accuracies in the order of 1/10 mm are therefore satisfactory. Moreover, as the connection is made via soldering and the solder used during the soldering phase is liquid and pliable, it is easily possible to even out any small deviations, with the surface tension of the solder also being used for precision correction work. In the case of connectors with electrical contacts, tolerances are evened out thanks to resilient contacts. In both cases, the contact surfaces can be as large as required, because it is possible to create a contact on any part of the contact surface.
With the availability of high-speed electrooptical converters, it is desirable to be able to use optical connections as well on a printed circuit board, as these connections exhibit a greater fault tolerance at very large bandwidths. However, because the optical waveguides to be used have cross-sectional dimensions in the order of 100 μm, the electrooptical converter must be positioned on the end of the optical waveguide with an accuracy of approximately 10 μm. To this end, connectors known in particular as mechanically-transferable (MT) connectors were created for fiber-optic connections, as described for example in the article by T. Stake et al, MT multifiber connectors and new applications, pp 994–999 of the 1994 Proceedings 44th Electronic Components and Technology Conference, IEEE New York 1994. In this process, high-precision hollow cylinders with an internal diameter of 0.7 mm are used in these MT connectors, and corresponding pins are secured into these cylinders, in order to laterally fix the position of both connector parts to be connected. To this end, optical cables and connectors to be added to the optical cables can be produced using optoelectronic converters, with the lateral positioning of the optical fibers and/or optoelectronic converters being fixed during the production of the connector thanks to corresponding mechanical precision.
One solution for coupling optical fibers embedded in a printed circuit board is described in the article “High Precision LIGA structures for optical fibre-in-board technology” by A. Picard et al, pp 77–80 of conference report EuPac '96, DVS Verlag, Düsseldorf 1996. According to this solution, optical connections that are embedded in a printed circuit board are integrated with a connector mounted on the surface. A costly process is involved in order to mount this connector, with the bared fibers having to be routed to the surface at an angle of 14°, sectioned, inserted into the connector, polished at the end face and glued in place. Apart from the high production costs, this solution assumes that printed circuit boards with inserted leads are used, so that the optical fibers can be laid using the same device. However, it is desirable to have a solution which enables printed circuit boards to be produced using printed circuit technology. It should be possible for the optical waveguides to be interconnected just as easily as for the electrical contact surfaces, without having to prepare each individual waveguide. To this end, an optoelectronic converter must be coupled to optical waveguides embedded in a printed circuit board in such a way that the lateral deviation is clearly less than the cross-sectional dimensions of the waveguides.
The solution is based on the consideration that it is not necessary to route the optical waveguides to the surface. Rather these optical waveguides can be completely “buried” in the printed circuit board. Immediately prior to assembly, slits are milled in, which bare the optical waveguides and the embedded reference elements parallel thereto. Optoelectronic couplers, which at the surface are also adjacent to corresponding solder terminals, are then inserted into the milled slits. It is preferable to use hollow cylinders as reference elements, into which suitable pins on the couplers are secured, as described in more detail in the exemplary embodiment.